Hi, I have a 91 GTO which has been running great - up until now!
Recently I found the battery flat and assumed it had just laid too long, however when I have trickle charged the battery and replaced it I found that the sidelights were on, despite them having been turned off at the steering wheel stalk. Even when I take the keys out the ignition they remain on. Looks like something electrical going wrong!
Do you have a foglight switch? Check to make sure it’s not been depressed, causing the sidelights to come on as well. As Max says also, and maybe a good look at your lighting/indicator stalk.