From the Owner's manual for 92 600SEL page 30:
"The red indicator lamps on the receivers should blink. They stop blinking after approx. 3 sec. when the vehicle is properly locked.
If the door or the trunk is not properly closed after you attempt to lock the car by remote control, the red indicator lamps will continue to blink (max. 10 sec). Open the door or trunk lid, close it properly, and lock the car again".
I had that veryfied on my car and it worked as described in the manual. If you sure you doors and trunk are closed properly, you need to check microswitches and wiring. I would start from the trunk because you have trunk handle popping out.