I can't tell you what letter goes with what number, and I won't have a chance to crawl in behind the dash to find out till Sunday, but, in the meantime, check out www.type2.com/library/electrip/bosterm.htm. That will, at least, give you the meaning of the Bosch terminal designations, and might get you half-way there. Maybe you could figure out the nature of the switch terminals with an ohm meter. BTW, is that the original switch.? My 544 manual schematic shows the same numerical designations on the switch as on the wiper motor. Their sequence clockwise looking at the terninals is: 54d, p,54, 31, and 31b. 54d On the switch goes to 53 on the motor, 54 on the switch goes to 53a on the motor, and P on the switch goes to the winshield washer. Hope this helps. If you solve the problem by Sunday, let me know off-line, so I won't have to crawl under the dash.
Bob S.