I'm not sure about your '90, specifically, but let me offer two suggestions that I've used in my '93 and '84 (and earlier cars) -- one of these will certainly fit:

On my '93, there's a dedicated conduit (meant for an optional cruise control, which I don't have). If you look under the brake's master cylinder, you might (if your '90 has it) a plastic conduit, ribbed, with a conical rubber cap. If you pull off the cap, you'll find that it offers a 1/2 inch internal diameter conduit into the passenger compartment. The latter's opening is high up (bottom of dashboard level) on the firewall, above your left foot's footrest. To see it on a '93, you have to remove the knee bolster, and then the under dash fiber cover. It's best to snake a stiff wire in from the passenger compartment to the engine compartment (the snake goes smoother in that direction), attach your wire (for the switch) to the snake, and then pull it back into the passenger compartment. Be sure to run the wire through the conical cap first, so you can replace the cap and prevent engine compartment fumes from going into the passenger compartment.

On my '84 and all earlier cars, there was some rubber "plugs" on rubber pads on the engine compartment's firewall. You can push an ice pick through the plugs, and then push your wires through the small hole. Because the hole is in rubber, I've found that the rubber closes tightly over the wires, sealing out fumes from coming into the passenger compartment.

One of these should work for you.