Where is the main power connection that goes through firewall on a 1998 Pontiac Transport Van?

There isn't a single "main power connection" that goes *through* the firewall on a 1998 Pontiac Transport van in the way you might be thinking (like a single massive cable). The power distribution is more complex. Several different wires carrying power to various components pass through the firewall, usually via grommets or rubber seals to protect the wiring. These include wires for:

* Battery power: This is distributed to the various circuits and components *after* passing through the firewall. You won't find a single large cable solely dedicated to this purpose. Several smaller cables carrying power from the battery will connect to various components on both sides of the firewall.

* Ignition system: Wires for the ignition system also pass through the firewall.

* Engine control module (ECM): The ECM receives power and sends signals through wires passing through the firewall.

* Sensors: Various engine sensors send information via wires through the firewall.

To find specific wiring related to a particular component (like a particular fuse or relay), you'll need a wiring diagram specific to your 1998 Pontiac Transport. A Haynes or Chilton repair manual for your vehicle would be invaluable. These manuals typically show where the main wire harnesses pass through the firewall and may show individual wire routes.

Without knowing the specific power issue you're trying to troubleshoot, it's impossible to pinpoint a single "main power connection" through the firewall. You need to be more specific about what system you're working on (e.g., headlights, radio, engine).