Funny I just did this to my 95 Firebird, Take out the glovebox and the stuff under it and you should be able to reach the Firewall from there, just drill your hole there.
The lower right side of the firewall. There is a wire harness grommet already there it's tight but I got a 0 gauge wire through by greasing the wire first.
The 1987 Pontiac Trans Am shifter knob is held in place with a retaining clip below the shifter knob. Push the retaining clip together and the shifter knob will raise off.
On the driverside firewall is first place I would look
Take the interior panel off that is located below the steering wheel, held in place with two screws.
Another fellow had mentioned running his wires where the hood release cable goes through the firewall. He pulled the rubber plug out of the firewall, ran his wires through, and then pushed the plug back in place.
A 78 Trans Am was available from the factory with three different engines. Two different versions of the Pontiac 400 cubic inch. both of them are denoted by a 'Z' in the 5th place in the VIN. The other engine that was available was an Oldsmobile supplied 403 cubic inch. This engine is identified by a 'K' in the fifth digit in the VIN. If the car has a manual trans it does NOT have the 403, if an auto trans it could have any of the three engines. The 403 was the only choice for a car sold new in CA or high altitudes.
If you are trying to add something to a Taurus the only easy place for wires to run through the firewall is through the speedometer cable boot. You can easily push wires along side of the cable.
Look for a short in the power wire to the computer. A place where the insulation has been cut through by a sharp edge, or there is a problem with the computer, bad ground. If there is specific condition when this happens, only when it rains, look for water leaks around the fuse box or the computer.
job done...motor comes out the top, leave the trans in place.
pop out old one and insert a new one using a block of wood as to not damage it and use a hammer to tap back in place...thats it
You will need to drill a hole large enough for the cable being very careful not to damage anything on the other side of the firewall. You will need to purchase a rubber grommet to insert into the hole and then run the cable through to protect the cable. You only need to run one cable, as you can ground the negative cable near the battery. Why are you running a battery cable through the firewall in the first place?
In the engine compartment is generally where they are at with the words Trans Fluid on the handle. If you have one it will be some place around the Battery or Air box. But some car's like a few Pontiac's do NOT a Trans drip stick, they have sealed units. I would suggest you consult your local Mercury Dealership. Hope This Helps.