I used back-up lights from an early 50s Chevy car for my '53 GMC. I rarely see those on ebay and NEVER see GMC park lights for sale.
Coincidentally, I was at the junk yard Saturday and as I was walking past an old MTA passenger bus I happened to look over and see a marker light on the side of the bus that looked identical to the parking/turn signal light on my '53 except that the lens was amber. The housing itself is plastic, but you cant see that with the lens and ring on it. The guy told me I could have anything off the bus I wanted. If that is a stock part on an old passenger bus it might be a good place to look for replacements for our old GMCs. Of course that might be the only bus with those lights on it.
