A 7 way bypass relay for vehicles that have a bulb failure system. Also includes an Audible relay for 12v (negative earth) vehicles to give audible warning that the trailer/caravan indicators are working.

It is a legal requirement to have a device fitted to the towing vehicle that warns the driver if the indicators on the trailer/caravan are not working 

  • Easy to replace relay if current one is faulty
  • Fully electronic bypass, low current draw 1mA
  • LED trailer lamp compatible
  • 7 way plug in vehicle signal lead
  • Indicator tell-tale output is a 2 way plug to power external LED or buzzer
  • Automatic cut out for individual circuit overload
  • Built in audible buzzer for the monitoring of trailer indicator circuits
  • 7 x screw terminal outputs
  • Screw terminals for +12v and 0v inputs
  • Comes complete with instructions

• Suitable for 12 volt negative earth vehicles only.   

• Examine the car carefully to see that all electrical circuits are working correctly and that no warning lights are showing. 

• Follow the vehicle manufacturers instructions, decide whether to disconnect the car battery, or remove fuses. Take into account: Would disconnection disrupt memory circuits, alarms, engine management, audio etc. (You may need a device to maintain these circuits if the battery is disconnected. 

• Can you make the necessary electrical connections safely with the battery connected? 

The relay is not a waterproof device and should be fixed in a dry ventilated space in a position providing protection from physical damage and close to the trailer socket. Drip loops should be formed in the cables connected to the relay to avoid water ingress. 

When wired as below the relay switches power directly from the vehicle battery to operate the trailer lamps and draws typically around 1 milliamp from the vehicle lighting circuits, this current is not detected by vehicle monitoring or switching devices.   

1. Locate the wiring to the rear lights of the vehicle (usually on one side) and select a suitable point at which to mount the By-pass relay and make the necessary relay and 7 core cable connections. Connections must be made to conductors carrying a 12 Volt supply to the bulbs and not to data cables. 

2. If access for the socket cables is not provided, drill suitable holes in the boot floor, close to the socket mounting point on the tow bar, taking care not to damage wires, pipes or vehicle bodywork. Remove any sharp edges from the holes with a file, treat with rust inhibitor and fit suitable grommets. 

3. Feed the cables from the towing sockets through the grommets to the area selected for location of the relay and secure. 

4. The White wire of the socket cable must be connected to a good earth, connection should be made to the vehicle chassis or bodywork, a good connection is important which should be bare metal, free from paint or rust. 

5. Relay Terminal “+12V” - If the vehicle manufacturer provides a spare fuse way suitable for towing electrics, this should be used. Alternatively run a power supply cable 28/0.3 (2.0mm2) from the battery to the terminal “+12V” of the relay, fitting a 15 amp in line fuse holder close to the battery, this supply should not feed any other circuit. The cable should be routed where it will not be cut or crushed, particularly attention should paid to places where the cable passes through bulkheads etc. (do not insert fuse until the installation is complete). 

6. Relay Terminal “GND” - Connect to a suitable good earth, connection should be made to the vehicle chassis or bodywork, a good connection is important which should be bare metal, free from paint or rust. 

7. The 7 long signal wires of the vehicle signal lead must be connected to the individual vehicle lamp circuits avoiding any data wiring or other devices as shown in the table below. Identify the function of individual vehicle rear lamp wires by tracing back to the bulb holder or using a digital meter to detect 12 Volt supply. 

8. Check all wiring connections and if correct, reconnect the battery or replace fuses and connect the auxiliary lighting. Check that both the vehicle lights and the auxiliary lights function correctly. Auxiliary indicator lamps should flash in unison with vehicle indicator lamps and the buzzer / panel lamp should operate. If any lamp fails to operate, check all wiring connections and the bulb. The buzzer will not operate if the auxiliary lighting is not connected or if the auxiliary indicator lamps fail.   

9. Relay output “BUZZER” - The relay is equipped with an internal buzzer to provide an audible signal when the trailer indicators are working correctly.  In addition a dashboard warning LED repeater light or an external piezo buzzer can be plugged into the terminals on the relay marked “BUZZER”.   

10. Bulb failure warning lights will only operate if a fault occurs on the towing vehicle. With the exception of indicator circuits trailer lamps are not monitored. ​

A HIGH QUALITY PRODUCT FROM MAYPOLE-  "The UK's Leading Towing and Touring Company"

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VECO aim to carry all advertised items in stock, however availability changes constantly. If the part(s) you desire are not in stock on the day the order is placed, we kindly ask you to allow 3-5 days for receipt of the goods. We aim to dispatch all orders within one working day of the receipt of payment if received by 4pm.

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Post Codes in this bracket are:

BT     IM     KA27-28     PA60-78     PH49-50
GY    IV     KW            PH17-26     TR21-25
HS    JE     PA20-49     PH30-44     ZE

Please contact us directly for postage quotations for these destinations.

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14 Gentleshaw Lane
West Midlands.
B91 2SS

Please note: we are not able to offer a refund for cut-to-length or made-to-measure items such as cable, sleeving, tubing, bespoke kits etc. unless the goods are damaged/faulty/incorrect. Unwanted goods must be returned in their un-opened, original packaging (where applicable). Please remember to include a copy of your receipt with details / reasons of return etc so we can process the return as promptly as possible.


  • Product Code: MP389B
  • In Stock. FREE Delivery in the UK
  • £23.70
  • Ex VAT: £19.75