Personally, I would buy some different coloured reels of wire, some crimps, a crimp tool, run those wires from the radio to the unfused side of the fuse box (ground wire to the chassis) and do the job properly.
It's very easy to get access to the type 2 T3's fuse box. Just undo 3 bolts and carefully flip it downwards.
From the radio end:-
Brown to chassis
Red to unfused side of fuse 7 (radio) or any B+ (battery live) terminal from the fuse box
Yellow to unfused side of fuse 10 or any switched live terminal from the fuse box
My VW rides:- 1973 1300 Beetle, 1982 2.0 aircooled Ci Autohomes Komet Type 2 T3 Camper
My non VW rides: 1985 Reliant Rialto 2 Estate, 1988 Reliant Rialto SE Hatch, 1989 Reliant Rialto SE Hatch