I have to replace the front engine mount on my '99 SLS.
Does anybody know if you have to replace all the mounts or is it alright just to replace the broken one. Please let me know if you've had the same problem.

You just replace the broken one(s).
It's not too hard... if you have a lift.
I'd hate to do one on the ground.
Good luck.
They break more often than you might think. You'll probably get one that looks a little different than the one you have.

i Just got both of my engine mounts replaced in my 2000 sls(the rear transmission mount and the front engine mount) ....I paid about $102 for the front mount at the dealer and i had a shop do it for $74.. and the rear mount was like 100 i think and like 50 for labor. I watched the the guy take the front mount off....It looked pretty difficult. took about an hour and he had to disonnect the back engine mount and just unbolt a bunch of stuff and raise the engine up pretty far. I know I wouldnt have been able to do it at my house. on my 94 seville I remember that front mount being very difficult to remove even after I got the front head off and when i had the front head back off, It was nearly impossible to get the bolts that go to the head lined up. But i ended up droppin the whole front end outta the car to put a new engine and trans in. and i did all that in my driveway lol. so , if its anything like the 94, I would have a shop do it.