Electrical issues

Year? Model? Options? 100% original? If not, what's been done to it?
Switch or miswired.
Switches and wiring changed from year to year.

Almost all A-bodies:
Key On, engine will show slight discharge from battery to power ignition, brake if on, and at least some oil indicator.
Key off, ammeter should show no discharge unless brake is applied or lights are on (including overhead dome).
Wipers and turn signals are usually powered from the 'accessory' circuit from the key switch. Wiring gets a little more complicated toward the end of the run.