rear main lights not working

Hello guys really need some help, I was recently having issues with my vdo gauges , when the car was off they will still be working, and when I turned heater on they will drop regardless off the car being on or off, so I bought new cables for the battery and the earth strap on the chassis leg and gear box and sanded the connection points back to bare metal and connected them up in still having issues but now my rear lights work on side lights but not on main lights, this is same with the rear number plate lights, but the rear fog lights come on when on the main light and the fog light switch is turned on, however the headlights are HID and work on main lights and side lights anyone know what it could be really confused and checked the engine bay incase I missed out the wiring all connected are clean I must admit I haven't checked the ground crown on the fuse box, but the car hasn't been driven in like a month or more so unsure weather this could be related, all help appreciated in advance

I would say a DVOM will help you here. (Digital Volt Ohm Meter)

If you have the lights on, then you should have 12V(battery) on the power pin of the rear lights.
If you have 12V to them, then the issue is bad grounds.
If you don't have 12V then the issue is a broken wire to them or a bad fuse.

I would also suspect a bad light switch, but testing for proper voltages is going to be quicker.

Do you have brake lights on the rears as well?
 

Thanks Briano sorry to sound silly, but how do I test for 12v at the lights is it out one probe on the ground of the lights and the other to  the light connection ? Hope i got this right  

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Haven't test the rear brakes yet but will check those out to its a double filament bulb so would think brake and rear lights are on the same bulb

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You can use the Back side of the socket and place the probe in there, turn the lights on, and use the negative lead to Frame ground (Bumper bolts)

I had an issue on my Cabby, where the rears acted the buttkiss…. I ran new grounds for each side to the bumper bolts and that fixed my issue.

what year and model are we talking about as your pic shows a 1hp model?  

Hello Briano it's a 1.6 gti 82 tintop, I will have a look over weekend and see if it helps all these issues started when it wad parked up so I'm wondering why all of a sudden the ceramic fuses all look ok

gremlins…. I have a bentley that goes up to.79 but my haynes only covers westy's.

Voltage testing is gonna be your saviour……
find out which you are missing, if the fuse has voltage on both sides, then you cdon't have voltage at the rear broken wire….

If you have voltage to a frame point on the rear to a different ground, then you have a ground issue at the rear.

Thanks Briano will test the ground on the rear, can you explain how to test voltage at the fuse thanks

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I would suspect the fusebox, or at least the connections there, i would buy some electronical  cleaning spray and take the connections off one at a time, check for water ingress, spray the connections let it dry, then reconnect and move on to the next.
But you may need a new fusebox.

BAZ-GTI said

Thanks Briano will test the ground on the rear, can you explain how to test voltage at the fuse thanks

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Turn the key to run.
Switch the lights on to park, or full on.
Measure the voltage at the top of the fuse to ground, then the bottom of the fuse to ground.
You also want to break out the bentley/haynes and verify that your switch is good as well.
 

Thanks Briano, I have the old style fuse box ceramic fuses, does anyone have a picture of the layout of this fuse box so i know which fuse it is thanks

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