95 v12 Jaguar, stereo wiring

My mother in law has a 95 Jaguar XJS v12. She likes the Eclipse AVN7000, and would like to place a navigaiton unit in the car. I know that it has a Harmond Kardon amp, to power the speakers and subwoofers. What I haven't been able to find is any information on the HU.     She wants to keep the factory amp and speakers if possible.     Does anyone know if the crossovers were active in the HU or amp or passive after the amp?    I imagine the HU is too old to be using the digital signal, but is it a high level speaker wire or low level pre amp signal?      My other concern is if the car uses the positive ground or negative ground.     I have read that Jag used both on the 95 models.     I have very little memory of the positive ground systems.    If used, is the positive ground based from the battery of the car?    Could I simply run the new HU's ground to chasis or directly to the battery?      I know that the high level speaker wire runs off AC and should be able to switch polarity.      In a  positive ground system is there a difference in the high level signal sent to the amp?      I guess I would just like a better description of the positive ground systems.     Once again, I have very little knowledge of them.     Thanks, Mike Gettinger.