anyone got any thoughts please ?

Old BMW E39, fwiw

had a towbar fitted about 3yr ago, pushfit connectors (ie cheapo) but worked fine for 2yr +

now I get an intermittent fault on the dashboard alert thingy, suggesting that one or sometimes both rear brake lights are out. it used to be a total lie and both would still work, and driving for a bit would make the warning lights go out. now the LHS light is on permamnently and the brake light really isn’t working (bulb is OK) any of the time. Still get right side warning light coming on – still intermittently only.

If I plug a lightboard into the towbar socket, the lightbar brake lights do come on and dashboard warning lights don’t alter.

Have checked for loose connections as best I can & none found. no obvious corrosion at sockets

so then, WTF ?!

I find most brake light problems of this ilk are down to a poor earth.
You may end up having to run extra earth cables from your rear lights.

+1 for poor earthing.

Also check that the actual bulbs and their holders in the light cluster aren’t corroded and any connectors associated with the tow bar.

As soon as I read the headline of the post I though ‘poor earthing’.

I suspect the fault lies somewhere between the lamp and the junction where the cables supplying it meet the connectors for the wires that latch on to the loom for the towbar light socket.

First meter out the earth between the lamp and the chassis for cintinuity. If this is OK inspect the sockets and lamps and meter the lamps for continuity.

Then find the switched line for the brake light. Chase it back using a multi-meter. Check continuity and for voltage on this line with the brake applied. Test for voltage at all identified points including the pin out on the socket which drives the light board lamps.

Make sure the socket is actually wired correctly.

Check for any inline connections in the loom and ensure the terminals are free of corrosion and put them back so they are seated properly.

definitely earth problem or problem with the scotchlocks (pushfit connectors)

These are rubbish and can slowly deteriorate over time losing connection, and as they slice through the existing wire moisture can get in and corrode the wire itself. Personally i would take the connectors of and (preferably) solder and heatshrink them or crimp and insulate them

This might be all the problem is

+1 for poor earthing.

AKA, “Ford Electrics”

Personally i would take the connectors off and (preferably) solder and heatshrink them

+1

And dab on some varnish to seal the heatshrink if you are feeling particulaly fastidious (’tis what we used to do when i was in the factory assembling military grade electronics).

Thanks, I agree, earth seems likely (I investigated the “final” route to earth, which is separate for each side and clean/dry)

Will have a go at the scotchblock – (I’m also suspicious of that which is really why I mentioned). I have already pushed & pulled at the wires where it attaches & “seem” intact but will try to get it off

(so you all think haunting is out of the question, then ?)

Supernatural forces update:

Half-disconnected the scotchblock – pulled off the towbar wiring “plug” but block still attached to main wires. Was all a bit dark where I was so decide to move car into the sunshine. WHen I got in, all warning lights were off and, yeh, lights now working. Decided to call it quits at that point but thought I’d just have a try with the towball plugged back in:

ALL lights now work but the dashboard thingy now thinks the RIGHT light is faulty (was left before that was actually not coming on at the light cluster). I’m leaving well alone. 😳

ALL lights now work but the dashboard thingy now thinks the RIGHT light is faulty (was left before that was actually not coming on at the light cluster). I’m leaving well alone.

I’d still try and sort out what is causing the problem.

+1 for poor earthing.
AKA, “Ford Electrics”

nah Pug and Citroen wiring makes Ford stuff look well considered…

ps scotchloks are the spawn of satan

I’d still try and sort out what is causing the problem.

So would I – but I CBA unless it goes off again. Getting rid soonish anyway
Cheers anyway to all involved

nah Pug and Citroen wiring makes Ford stuff look well considered…

And Alfa electrics make Pug and Citr…

You will take a hit on s/h value if there is a warning light on, surely?

worth next to nowt molgrips 🙁