Liquid electrical tape (or Dip-Grip) or even RTV will provide good sealing.
Heat shrink with the hotmelt adhesive inside provides a better seal than standard heat shrink but it's tougher to find.
Moisture wicking is a big problem with terminals in engine bays and under-car areas. Some form of sealing is a must.
Actually a great wire to use is TNN electrical wire. it has a thin nylon outer jacket over the PVC. If price is no object you could use the teflon coated, silver plated, Arospace - Nuclear plant grade wire.
Soldering is the ultimate way to ensure good contact, make sure you clean off the flux though, rosin flux is corrosive at higher temps.