306 equinox diagnostics light ON

I just had a new back and middle box fitted to my 1998 Peugeot 306 equinox. But the diagnotics light has come on and the lights,horn indicators,wipers etc., will not switch on. The engine is turning but will not start. Telephoned garage and they think its a short in my wiring as there was a waer dripping from my inside light as sunroof was left open during the rain, Does anyone recognise the problem