Speedometer lights are trying to come on...This is different than what I've seen posted??

Without depressing brake pedal, pushing Start button once turns on front touchscreen. If start button is then pushed again the back speedometer dash tries to turn on for just a split second but then both screens go off. If I do depress the brake pedal and try to start the vehicle the back speedometer dash tries to light up for a second but then goes out. The aux battery is good (about 13.2 volts) and the Main battery is I believe still good too. I work out of my car and I have an inverter in the back of the car to charge tool batteries and I recently discovered the connections on both the inverter and aux battery were very loose and I realized that's why I've occasionally had to jump the vehicle (from the front jump posts), I've since tightened everything as well as tested the aux/accessory battery.

Any ideas? This car is great. 2004 Gen 2, half a million miles (in fact, we've recently bought an 05 as a back up). But the original 04 has still been going strong with almost no major problems all this time.