I'm guessing that it may be the voltage regulator sticking, and letting it drain your battery.
I went to AGCO partsbooks, and looked up the wiring on the 202. It does show a pretty decent exploded view of it, if you don't have a OEM service manual. Here is the link:
AGCO Parts Books
Enter as a guest user. When the next page opens, type in 202 in the model, and select Massey Ferguson to the right of that.
The next page will open with a parts list. Scroll down, until you find Electrical wiring.
Next page will show an exploded view of the wiring. Right click on the picture, and it will give a little menu to zoom in. To move around on the picture, left click and hold down the "Alt" key ( Just right of the spacer bar)
In the parts list, to the right of the picture, will describe which wire is for what..
No color codes, but does show from what post on the Genny, to what post on the regulator, and so on.
Hope this helps..!!