I can't tell from your description whether or not the thing is an antenna signal booster.

I do know that an antenna signal booster would not have RCA cables on it, unless the previous Mk1 owner had soldered his RCA grounding adapters to the ground wire on the signal booster.

I'm digging through the empeg photos to locate a picture of the signal booster that shipped with the Mk1. Note that this would be completely unused if your car came with such a signal-booster built-in (such as some VW and Audi cars do).