Greetings From Arench!
Please tell us which engine you have.
Run your tack signal wire carefully through the firewall to the distributor. Make sure the wire is protected because if this wire shorts to ground later down the road the truck will not run.
Connect the wire to the distributor cap terminal marched "tach" If there is a wire already in the terminal then splice in your tack signal wire to the wire going to that terminal (usually a white wire).
The photo here shows a guy placing a blue female spade terminal in the distributor cap terminal marked "TACH"