Car audio wiring is very simple really. In fact it is so simple that people often overlook or neglect things - and that can lead to a lot of problems. :)

You just have to take things one step at a time trying to figure out problems. Your constant line should be near 12v with no key inserted into the ignition. Your accessory line should read 0v in the same scenario. Turning your key to the accessory position should now give you about 12v on each. Starting your car should (could) give you nearly 14v on each (YMMV).

You can use a simple test light to check those two lines. If you want to be ultra-safe when probing in your dash, you can use a multimeter (airbags can be deployed when completing a circuit with a test light - obviously when touching the wrong (tm) leads. :)

Bruno