My 1999 300M won't start. Turn over good and I can hear the fuel pump come on. But it doesn't fire. Maybe timing belt broke? It was leaking antifreeze ocassionally recently and I'm wondering if the antifreeze weakened the belt and caused it to break. Where or what would I check first?

 

For a '99 model year and probably over 100K, with a leaking water pump. Yes it is likely the timing belt broke. There should be a plastic cover over the TB area that can be easily removed to see if the belt is in one piece or not. Hopefully it is OK as these motors are an interference motor and will damage valves and pistons if the TB breaks or is out of "time".