Sorry N. Hodges, I forgot to respond in my previous message.

Your problem is most likely a ground issue. MB thought it was a good idea to connect a hundred ground wires to one bolt behind the instrument cluster, tied together with one locking-type screw. Needless to say, some W140s have bouncing ground issues. Mine was OK, but you will have to check yours.

It flashes because power has temporarily floated, which forces the microprocessor in to an unknown or reset state. That flashing is the chip completing a POST. Try it with your seat belt off and you should see the friendly seat belt warning flash a few times.

Remove the instrument cluster using two hand-held hook tools from your local HW store. Yes, there are no screws holding the cluster in. Make sure to lower the steering wheel down and forwards all the way and that the ignition is off. There are two hook type plugs, which you'll need the hook tool again to pry off, and two wierd round plugs. (I assume you have a W140) Once you remove them you can pull aside the cluster and investigate the now empty cavity. If the ground is OK, it could alternatively be a bad input or a bad component on the board.... in which case you can call me !
but yeah, electrical problems, esp. intermittent ones, are tough ones.