No expertise in that particular model.
Generally, a good voltage reading under low load, followed by a zero voltage reading under load means a marginal electrical connection somewhere, often a poor ground, or dirty terminals. Start by wiggling the fuse, just to be sure, then clean and grease the terminals and ground(s). If all that fails, suspect the relay.
Recently had a similar problem on my boat, which turned out to be a hairline crack in the positive battery terminal plate. Very unusual problem. Sure was tough to track down!
1986 300E 5-Speed 240k mi.