So i started off with pretty much a bare shell before easter, and built up this early '85 model using some parts from my late '85, which itself used some parts from my old '88. This time i wanted to keep it as original as possible but this did include the '88 electrics - anyone who knows these cars will know that '88 electrics are quite different to '85 electrics. 

 

I did use the '85 rear loom as it includes a rear washer bottle. At first i had a few teething problems but all was well by the time it came to MoT the car. Since then though, the passenger side tail light has been struggling to hold on for dear life. It works, atm anyway, but it's nowhere near as bright (or dim) as the driver's side.

 

I've tried every connection. The one near the bonnet-pull had a couple of incompatibilities but i ironed those out. The '88 fuse box was tarnished but i've given the contacts a clean and tried new fuses. I've tried a different rear bulb carrier and bulb. Nothing seems to change anything.

 

The registration plate lights are even. Both tail light sockets are getting a healthy 12v and it doesn't make a difference if the engine is running or not.

There's having voltage and having a decent flow of current.

 

If you want the bodge answer that will tell you if it's the loom, the unit or the earth: Get a length of wire with a spade connector on each end and run it between the clusters using the terminals that are on the back of the light units for plugging a towbar in. Can't remember which pin it is off hand, but a multimeter of test light will tell you.

 

I've had a fair few random rear lights issues on these over the years that I've never quite managed to pin down (normally numberplate lights) and pretty much always end up tapping it off another light.

 

Only downside is, your parking lights will light both up for one side and none for the other.

I'm not fussed about parking lights, though i'd naturally prefer to get the original circuits flowing correctly. As you say, the voltage may be there but it's not a strong current. Another way around it i considered was low power LED's, but as the tail lights are only 5w anyway there's clearly a major restriction going on (or more worryingly, a short/cross circuit).

 

I want to take the carpet out anyway, as i have a better one to put in, so i'm gonna have another look when i do that.

You have both hit the nail on the head with the voltage v current statement - You obviously need battery voltage there, but you also need conductors capable of carrying the current, even though your 5 watt bulb will draw less than 1/2 amp, you still need the capacity to carry it.

If you care to rig up a test light (a bulb-holder, complete with bulb, connected to two lengths of wire) you can then connect that to a known good positive feed and check the earth connection by placing the other wire on it.

One less likely possibility, is that you have got a 24 volt bulb (as used on commercial vehicles) instead of a 12 v one ( the voltage will be printed on it.)

 

Regards