[ischmitz]

sometimes a simple Digital Multimeter helps.....they can be had for less than 10$ at your FLAPS or on evil bay.

Measure the battery voltage by touching both lead poles while an assistant cranks. If the voltage drops to below 8V you know what's going on. The starter has some pretty nasty power demands and a battery that might appear perfectly fine when powering just some lights might be brought to its knees when the starter wants to draw 50 Amps or more.

Further, when you say you hooked up jumper cables you need to let us know exactly what you connected where. Let's assume you had another battery/car supply +12V to the battery of your 911 while it was installed in the car. If with that scenario you still had issues starting you might have an issue from the battery to the rest of the car and the starter in particular. As others said there is a ground wire from the battery directly to the chassis. All other consumers (starter, lights, etc.) itself have one wire going to chassis for GND. And the engine/transmission has a braided line (ground strap that ensures good electrical connection to the chassis. The positive big wire goes directly from the battery to the starter relay and on to the alternator. Other lines go to the fuse panel.

Ingo