As far as I know the wiring should be identical to the 924 turbo.
The 924 Carrera GT has the same wiring, its probable that the GTS is the same.
Go to 924board.org and ask there, or https://porsche924.co.uk/forum/
create an account and ask.
Both places are packing with 924 knowledge and a few of us own GTs or GTSs so your chance to fix the problem is highest on those two forums.
Anyway, is this car going to be used on the track? did I get that right? Your friend must be really rich as these cars fetch 200k + these days.