1986 Mercedes-Benz 300E - keyless entry remote won't open the car?

The 1986 300E (or any year of the W124 chassis for that matter) did not come with a remote fob/keyless entry. If you do have such a system, it is an aftermarket component and will need to be diagnosed by a person familiar with the brand that is installed. It could be a faulty component or a bad installation that is giving you the trouble.