Just take it to an auto electrician and ask them to diagnose the fault then once you know what it is you can decide whether to tackle it yourself or if it's a bit more involved than what you are happy to undertake get them to quote to carry out the repair. You never know it could be something fairly cheap & easy to fix but until you know what the problem is you will be going round in circles.
If you need any spares for your ST24 contact this fella on Ebay - yaf76, he is currently breaking 2 ST24s so may be able to help you once you know what the problem is. I have just bought a petrol flap release cable from him and he was very helpful and seemed a really decent bloke.