I have had to use pipe extensions on the allen wrench to remove these fittings from the differential on just about every Mercedes I have owned. At least the first time. I doubt you would need to use any heat, just get enough pipe to yield the torque required to break it loose. May need better leverage than lying on your back as Rich 126 said. Good luck!

__________________
1997 S600 Coupe
2002 Lexus SC430
2004 Lexus RX330 (wife's car)
Owned:
1993 500 SEL
1994 E320 (wife's car)
1987 300 SDL
1978 450 SEL
1980 300 TD (wife's car)
1969 280 SL