If the panel lights dim when you engage the starter, that is an indication of either a bad battery.... one or more cells not working, or poor connections. I would clean and tighten all connections from the battery to the starter and battery to ground. If that resolves the problem, it is a bad connection. If that doesn't, check each cable with an ohm meter to see if there is a bad battery cable. If the cables check out, the only thing left is the battery itself. If you have a known good battery, and install it in the tractor and it starts, you can by pass the check of the cable resistance. The only time I find bad battery cables is when the cables are old and there has been a lot of corrosion of the terminals.