I have a NOS 65-67 Chevrolet Stereo Multiplex adapter on Ebay. This is the early style with the rivited on brackets. The auction started at $9.99 with no reserve.
NOS Stereo Multiplex Adapter
If you want to check out a very rare unit, look at this 68 Firebird only Multiplex:
http://www.ebay.com/itm/261006739244?item=261006739244
I also have multiple original factory Delco AM/FM radios listed. All started at $9.99 with no reserve.
67 Camaro/Firebird AM/FM
65 Chevrolet AM/FM
66 Chevrolet AM/FM
69 Camaro, Chevelle, Impala, and Nova AM/FM
65-66 Corvair AM/FM
65-66 Pontiac AM/FM