This looks familiar to what I tracked down on my 89 XP recently. Looking at the wiring diagram in the shop manual for the 94 XP on page 07-01-5 this particular circuit looks familiar, maybe I can help regarding the starter problem at least...
As mentioned above, one of the big red cables in the box connects to the battery and the other big red cable connects to the starter. The black encapsulated thingy the big red cables connect to is the starter selenoid. There will be 2 other connections on the selenoid; one goes to the battery thru the 5a fuse (wiring diagram indicates this is YL/RD), you should read 12v on the connector post if this part is connecting ok. Actually you should read 12v on both posts if the 5a fuse is inserted.
The last connection on the selenoid (wiring diagram indicates BK/WH) will connect to ground when the safety lanyard is attached and the start button is pressed. To test, remove the 5a fuse (else you will have 12v thru the primary of the selenoid) and ohm this connection post to ground with both the safety lanyard switch and start switch depressed, you should read close to a dead short. The cheap and dirty test, once you have positively verified you know which connection is which, is to ground the selenoid lead going to the switches ... if you get the wrong one you will blow the 5a fuse.
In my case I found a bad wire-to-terminal lug connection on the ground lead going to the switches.
Let us know if this helps.