series/single/parallel wiring for G&L M2500?

I can't really say for sure because I've never seen a M2500 bass. But for the L2500 (assuming your pickups have 4 wires coming out of them) the colors of the wires on the L2500 are:

Coil closest to bridge is white = hot; black = ground.
Coil closest to neck is Yellow = hot; Green = ground (and Green MAY be connected to "ground " which is to say the copper plate under the pickup. This is important because it means that when connecting in series it must be the green wire and not the black wire that is grounded. The Standard "K-mod" switch wiring found online should work great. Only in your case you'll need two special 3-way switches instead of the large one G&L uses to switch both pickups at once. Mouser has them. Or you can use 2 of the G&L switches and just leave half of it unhooked.

Note that if you use the "single coil" mod, it needs to be wired as Humbucker when both bridge and neck pickups are balanced. To do that you must choose opposite coils in the two pickups. In other words the single coils will either be the "inside" coils (sounds more like stingray) or "outside" coils (sounds more like Jazz bass). In any case, just like a jazz bass soloing a pickup, a single coil will give you single coil hum. My favorite is "outside coils".

Good luck!

PS. I call this "Leo's last circuit" which I've wired into a number of basses both passive and active. Works great!

 

Too bad! Shouldn't be too hard though. Ge a cheap Ohm meter and you should be able to figure out which pair of wires goes to which coil (assuming you can't just SEE which is which when you look at the bottom of the pickups). In order to figure out which is hot and which is ground, You need a cable you can plug into your amp with just two wires on the other end. Try to find neck ground by using ohm meter to find is one of the wires is connected to the copper plate under the pickup. Since you already know "hot" for that one half the pickup is figured out. Next you need to figure out the other coil. To do that connect ground on your cord to the wire we just found to be grounded to copper above. Then take the two wires for the OTHER coil and tack one of them to the Hot of the coil just wired and the last wire to the "hot" on the cable. You are going to have to do this twice, next time reversing the two wires to the bridge coil. ONE of the two ways will have LOWER hum. THAT is the correct way the pup is wired. And the wire connected to the cable center is "hot" for the bridge coil. It all sounds more complicated than it really is.

Good luck.

 

Yeah, that's the basic wiring only pickups are often wound two ways. Sometimes the coils are identical (as shown in the diagram) and the wires to one coil swapped to cancel hum (reversed magents make string signals in phase). But this has a problem that when the top coil is in series the Hot wire goes to the winding cllosest to the poles. This means that if you touch the pole pieces without being grounded (touch strings) you inject hum right into the preamp. So a better way is to reverse wind the top coil so the ground windings are near the poles and less hum is injected. However, the way coils are wound is determined by the manufacturer and you can't change it, but it is always wise to ground the poles to stop this kind of hum pickup. On a jazz bass for example:

https://www.audereaudio.com/faq_pumag_gnd.htm

For some reason G&L never grounds their poles either so I used conductive paint on the underside of mine to ground them all to ground out the hum.

Yes it shouldn't be hard to figure out as the ground wire goes to the - of bottom coil. The connection between them is the top of the bottom coil (+) and the bottom of the top coil (-), And the output wire is the top (+) of the top coil. You don't have to worry about coil wind direction as this wiring takes into account whatever they chose to do. From that you'd have to make associations with the L2500 colors which would be Green for the ground wire and white for the output wire and the two together would be yellow for the + from the bottom coil and black for the - from the second coil. Once you have those colors associated you can then use a standard L2500 K-mod diagram to give you series/parallel and single coil. Should not be too much trouble to figure out.