GTI 1990 1.8 manual - starting problem

When I turn the key, the engine is very reluctant to start. It's just about OK if I run the engine every day but if I don't run it for say 3 days, the car is dead! and I have to recharge the battery. The car is regularly serviced, garaged every night and I've just had a new battery put in. The problem is still persisting. My trusty VW mechanic is stumped and can only suggest crawling over the electrics to try and track down the problem (£!!!!!!!). Anyone got any ideas where to start? And yes I do check that the lights and radio are switched off overnight!

Try checking the trunk light. A friend of mine had the same problem this sumemr and the only reason we figured it out was becasue we were doing body work on the back end and could see light through a rust hole we were patching.