Mk2.5 Issues Amber Engine Light & No Radio

[Renovation]

A friend let his battery die and replaced it - there is probably more to the story but that's all he's admitting to.

He called me after nothing worked and I found the main fuse had blown.

I replaced it and the car started and seems to run fine.

But the radio is dead (fuse is fine) and the amber check engine light is on.

I tried plugging in an OBD code reader under the dash but there isn't any power to the OBD socket so it won't read codes.

I've removed the battery but the light instantly reappears.

Any ideas ?

[Renovation]

No ideas ?