Where is it leaking?
Mine always leaked around the doors and there wasn't anything I found I could do to stop it. Sometimes bending / tweaking the door frames a little to put more pressure on where the leak was would help some.
My first one was a 1962 Jeep that I owned in about 1976 in Fairbanks. When winter came I mounted two more heaters in it, one on top of each rear fenderwell. It actually did rather well at -60F.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.