Battery Disconnection - Big Bills

[Delilah 62]

I have had to have some additional electrical equipment added to my Prius for work reasons. A simple item connected to the Battery with a switch inside the cockpit. The Auto Elec contractor disconnects the Battery to do the work and reconnects afterwards.

Long story short: On collection the ABS, PAS and '!' lights were on, plus the SatNav screen kindly said 'PROPLEM'. Car was low-loaded to Toyota who have found that it is the Anti-skid/Stability control box is faulty. Toyota admit is nowhere near where the Auto Elec bloke would have been working, but have refused to agree it is a warrantee item yet (and as they had to get Toyota UK to agree to their fitting the item before starting work, due to the huge expense, I think they'll fight it).

Listening to the contractor, he says he only disconnected the Battery and reconnected it again. His work on the car was minimal...

Has anyone else ever had any difficulties, or been warned not to complete simple electrical work on a Prius? The bill they are talking about here sounds to be £1000's and even Toyota aren't sounding surprised (although they are a bit confused)...