quote:Am I correct that the test light would not work without a proper ground ? If I did not have a proper ground at the studs is it possible for the other lights to work ?
Test light needs ground.
GRP 4 lights only ground at the one mounting stud. If you have some lights working then they have a ground and thus you will have all lights grounded, as the ground gets to all sockets through the flat metal piece all the sockets mount to. And YES, as I wrote very early on, a fine looking socket could have corrosion issues blocking the ground - but this is highly unlikely for only BOTH backups to have that problem.
Carello and Altissimo light plugs are wired the same, whether stock or GRP 4. Unplug the stock, plug in the GRP 4 and everything should be fine.
Have you taken a stock light and remounted it to see if IT had a backup light?
Remember, just plugging it in will not be enough, you will have to mount it and make sure it has a ground. I think this is the next step you should take.
Have you taken a KNOWN good bulb and swapped it for a back up bulb? Your back up and yellow signal bulbs are the same.
Stay with it and you'll find the problem. You won't need a three foot breaker bar, get your hands all greasy or have to remove the engine. Electrical problems are not hard, just time consuming.
Do you have, or can you borrow, a multimeter/voltmeter? Or a continuity tester?
If you want talk this out, I'm home all weekend. California, 559-281-3497
Larry