I always use Raychem mini seal splices but it is what I stock for my race car harnesses so it may be overkill but it comes with the confidence of military specification.
MIL-S-81824/1 Qualification
Voltage Drop 6.9 mV at 4.5 A vs 8.1 mV for an equal length of wire
Temperature Range -55° to 150°C [-67° to 302°F]
