Sounds to me like you've got some kind of short in your lighting circuit. Chances are you'll find it when you do the mods but of course if you do it renders the mod somewhat superfluous.
The thing about TLs now is they are all old and with that is bound to come all the interest that age brings. I'm the same but I'm just grateful that my gaskets haven't started leaking yet.
Chassis is pretty knackered though. These aftermarket parts are never as good as the originals.