I just did this with our 67 they are called "festoon bulb holder". I replaced the entire rear harness.
Those bulb holders are only available thru MH electrical. $11 each! I ordered 4 and it cost $31 in shipping so Canuck bucks is pretty damn close to $100 black is original, white is new. Check the wiring going to the holder. It gets brittle and is very close to metal where it can short out.
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