I have an aftermarket alarm system on my 95 C280.
It is a Viper 300 ESP. Unfortunately, I don't know the specifics of the install, because it came with the car when I got it.
It is generally a pretty good alarm. The "brain" is hidden under the dashboard, and the siren/speaker is under the hood on the driver's side by the fuse panel.
I have a remote that locks/unlocks the car, while flashing the parking lamps (arms/disarms alarm in congruence). I also have a valet switch under the dash which turns off the alarm (also eliminates chirping sound, but still flashing the parking lamps); essentially just a remote keyless entry (no alarm).
I was recently wondering why I could still start my car after locking (but not UNlocking) the car using the key... I learned from Benzmac or MBDoc that the alarm people had more than likely bypassed the factory starter disabler for the alarm system (which has it's own starter disabler).
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)