How many amps can I route through the automotive chassis?
Because the cross-sectional area of the chassis is so large, it provides an excellent return path for DC currents to the battery, even though steel is not a very good conductor. The steel chassis itself has a lower resistance than the copper cable carrying the positive leg current in the circuit. Also, using the chassis as a DC ground is common and accepted.

Just make sure the connection points between wiring and the chassis remain free of corrosion, and that there is good contact between wires and chassis. Don't rely on simple surface contact between a spade terminal or ring terminal and the chassis. There may be paint or an oily film on the chassis in some places. Remove any paint or other coating on the chassis before making contact. You may also need to use serrated washers under any ring terminals to ensure good contact.

The wires you use must also be adequate for 100A, which means something like AWG 00 cable. This applies to the chassis bonding wire for the battery, too. When you first put your system in service, measure the voltage drop from end to end while 100A is flowing, and check that the wires and connection points do not get hot after a few minutes of high current operation.