mk1 cabbys , caddys and golfs all suffer with similar probs due to water getting in the fuse box and making things happen all over the shop or not work at all....
not sure on getting a new fuse box as i dont have much to do with mk1's but you could try removing the old one (take pics and write notes olf what goes where etc...), then strip clean and emery all exposed connections and tracks etc then cover in wd40 and re assemble and see if that works for a low cost fix...