Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
I'd 1st try replacing the battery in the key-fob with new and then disconnect the battery and charge it up over night
then when you reconnect it hold down the unlock button on the fob at the same time
I'd also check the state of the grounding for alarm and ECU its the giant brown (grounding color in these cars) wire octopus located under your center console
OK if its of that era then try unlocking the car trunk with the key (as door micro switches may not work)
if this doesn't work then it sounds like the alarm unit may be faulty
you can replace it with another second hand unit from another SEL ?
alternatively this bypass procedure may help