test light!
How are the sockets and the contacts inside? sometimes they get corroded or pushed inwards to the point of not making contact.
Get a 12 volt test light and test for juice right at the socket. Hook the test light up to a good ground and have a helper activate the brake lights with the headlights off, and then with the headlights on. If the test light functions normally it's a ground issue, if it misbehaves in the same manner as your taillight, it's a hot-wiring issue (either somewhere in the trunk wiring or else right at the socket). Do the same test with the backup light.
If it's a taillight ground issue, since three of four lights are functioning correctly, we can assume the problem is the black wire leading from the socket to the wiring harness. Since these wires are tucked up well out of the way the main suspect is corrosion where the wire connects to the socket. If you're lucky, or limber, or both you may be able to reach up to the socket and *gently* wiggle the socket connections there....might be enough to re-establsh a ground, might not. Still worth a try before taking the bumper off. Note that the inner taillights are somewhat acessable thru the fuel filler door, the outer ones less so....
If it's a hot-wiring issue it might be a bare wire touching ground somewhere. Try *gently* moving the wire harness as your helper stands on the brakes, and see if the lighting issue changes any. If it's your lucky day the problem could be relved with no more than a quick application of some electrical tape If not, it's usually socket connections again. Usually can't fix hot-wire socket problems by wiggling but again, it can't hurt as long as they're not manhandled at all.....just a little wiggle, nothing drastic. If it's corrosion it might help at least in the short (excuse pun please ) term.
Same for the backup lights. Test light first, wiring harness second, socket connections third. If none of that helps the sockets may indeed need to be replaced which means dropping the bumper to get at them. Big job that.
I have a 1970 shop manual diagram (cutaway drawing) of the rear bumper lighting assemblies, can scan and upload next time I'm home if that helps at all. Let me know!
Good luck and hope this helps.
-Mike
1968 Special daily driver 285K "Roadmaster"
1969 Skylark 2dr 38K 99% original- gone but not forgotten
1992 Chev Lumina 3.4 project- just plain gone
and a '75 Ford Granada project for sale!
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