If you drill any holes through metal panels to run wires through, it essential to use some kind of grommet to prevent the edges of the hole from chafing through the insulation ESPECIALLY with heavy gauge wires connected directly to the battery (which should be fused too)

I believe the others are correct about there being extra unused passages in the wiring harness grommet. This would be the best way to go.

There's also a sizeable cutout with a rubber plug way off in the upper corner of the firewall behind the hood hinge/spring on the passenger side. You can't see it from under the hood; have to reach in with your hand. If you peel back the carpet on the inside, you can see it from the inside.

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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

I don't know if this is in your '86. It wasn't in my '84, but it's in my two '93s, so sometime in-between, they included this (maybe in '86 when they made a whole lot of changes).

Anyway, in my '93s, there's a conduit (a rubber, flexible tube) that runs from the engine compartment (the opening is just beneath the brake master cylinder) to the passenger compartment (this opening is just next to the hood release pull, but it's above the wiring shield, which you have to drop to see it). You can snake any number of wires through it (it's about 3/4 inch in diameter). I've been told it was to run a cruise control's wires, but it can serve you well if you've got the conduit.

Good luck.

Don't drill a hole. Check in the area round the brake booster and around the existing cables that go through the fire wall. You'll find a couple of rubber plugs in the firewall that will allow you to run the cable into the dash without drilling.