So you have an existing wire run you need to tap into, in the middle of the wire run.
Option 1: cut the wire, install ring terminals on both sides, install a terminal block, and add your wire.
Option 2: not preferred, but not too bad, use one of those 3 way crimp terminals.
Option 3: perhaps a copper split bolt to add the wire to with a wad if electrical tape to hopefully keep corrosion at Bay.
PO's option:. Don't cut the existing wire, just strip a couple inches of insulation out of the middle of the wire run, strip your add on wire, wrap the new wire around the existing, and add a booger of solder to hold it in place.
This was behind a bulkhead by the way, neither hot nor ground taps we're insulated.