looking under the firewall, follow the hoses and wires that go through the firewall. One will have enough room to squeeze the wires through. Make sure that you can reach the wires on the other side of the firewall as well.
The speedometer cable is likely bad
87 Monte Carloif 87 Monte Carlo has a cable type release clamp on a small pair of vice grips and pull hard.
connected to the back of the odometer and runs in to the transmission it pulls out
Try on the top of the starter - follow the + battery cable
Generally, yes.
unhook a battery cable for about 3 minutes and re-hook.
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?
Unhook it from the tranny and then the other end is up under the dash hooked to the speedometer, unhook it and take the cable out, just reverse the process for installation.
Remove the positive cable from your 1998 Monte Carlo battery. Remove the cables from the front of the starter. Remove the starter retaining bolts. Reverse the process to install the new starter.
Depending on what you mean by "repair", the common issue for a speedo not working in an 80s Monte, is due to a bad cable, which would just need replacing. Your question needs a little more detail.
In a 1985 Monte Carlo, the blend door cable is located under the dashboard, connecting the temperature control lever to the blend door actuator. To access it, you typically need to remove the lower dashboard panel. The cable allows you to adjust the blend of hot and cold air coming from the heater and air conditioning system. Make sure to check for any kinks or disconnections if you're experiencing temperature control issues.
cable runs through the firewall under the radio. Unclip both ends of the cable. Pull through, its easier to pull from the engine bay