Study that wiring diagram.  The plug and the pin number within the plug will tell you where the wire should go (same as what Daniel says) .  Trace out each wire, it will tell you a lot. From the transformer you should only have two spare wires with spade connectors (the sort that fit the bridge) - you'll have four spade terminated wires all up, but two of them are on you existing bridge.  Again have a look at the wiring diagram - can't stress how important / invaluable it is, a picture and a thousand words and all that.  On the one bridge you have in place measure the ac voltage, it'll either be 13 or 25 volts, the other two wires with spades from the transformer will have to be the other ac voltage going to the other bridge.

Also have read of the Marvin WMS repair guide - has a good section on power supply testing.