Freeze plugs are to plug the holes when the engine is casted. Most, if not all, engines have freeze plugs. 99% of the time freeze plugs leak due to rust and neglected coolant change. The rust inhibitor in the coolant prevents them from rusting and hence leak.

Some cars have freeze plug between the engine and tranny. Go figure how much it costs to fix in labor. Part is only $1 or less. If one freeze plug leaks then the chance the other will leak is pretty high.

Good luck.

http://www.automedia.com/Replacing_a_Freeze_Plug/ccr20070401fp/1

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.