If you're planning to use the existing bulbs in the S-10 you will need to use diodes as Dave says to prevent power from back-feeding from the S-10 to the MH and vice versa.
If you don't want to cut into the S-10 wiring and use diodes, you can install separate light sockets in the tail light assemblies of the S-10 as an alternative. These will be wired to the MH only and will therefore not cause a problem with back-feeding of DC power. Kits are sold by Blue Ox
. Kits with LED lights
are also available.