WR250 Z electrical issue

Let me start by stating I do not have first hand knowledge of your bike. I do have extensive knowledge of low voltage electronics.
NOTE: DO NOT USE A METER SET TO OHMS ON A HOT CIRCUIT, DAMAGE TO YOUR METER OR THE CIRCUITRY WILL LIKELY OCCUR!!!!
First off, your battery should have approximately 13VDC fully charged with no load on it. The lighting will likely get brighter as the rpms get higher. Typical of two strokes.
Secondly, there are two types of wiring schemes. 1 type of circuit is a ground seeking and the other a voltage seeking. While both types are simular, troubleshooting is slightly different. Using a volt/ohm meter you can easily determine which type of circuitry your bike has. Start by disconnecting the battery and turn the keyswitch on.
With your meter set on OHM's, place one lead on the frame and then the other lead to one leg of your head light circuit. If you have less than 100 ohms, you have a voltage seeking circuit. If you have no change, move the second lead to the other leg of your head light circuit. If you still have no change, check the third leg (high/low beam and ground). If you still have no change, your circuit is a ground seeking type.
Turn of the switch and re-install the battery. Change your meter to read DC voltage on a range that will register 13 volts or slightly higher.
1) Voltage seeking circuits. Do these checks with the engine off and then with the engine on.
Continued....

Check these circuits in order: LOW beam, High beam, TAIL light.
Meter across the head light bulb. You should read 12+VDC + or - 2VDC across two legs to GROUND but NOT the other. If no voltage is present, the engine must be running, start the engine and repeat. Turn the engine off after locating the low beam circuit. Repeat these steps on the HIGH beam circuit and tail light circuit. The LOW beam circuit should have no voltage when the HIGH beam circuit is on and vice versa. The tail light should have voltage on both. The common leg will either be tied together with a wire or will be grounded to the frame. If grounded to the frame, clean the connections. If tied together with a wire, check the full length of the wire for damage, repair as necessary.
If your voltage is 9 VDC or less, you likely have a Voltage regulator issue, replace and recheck.
BRAKE light and TURN signals. The BRAKE light should be a dual filement bulb. The second filement should come on when the brakes are applied. It should read on the meter at 12+VDC + or - 2VDC across one leg to GROUND. The TURN signals should alternate 12VDC to Zero while working.
2)Ground seeking circuits. These circuits should all read 12VDC + or - 2VDC. These circuits should all be HOT even if the bulbs are NOT shining. When the circuit is turned ON, they will receive a GROUND to complete the circuit. All of the same checks should be made as in the Voltage Seeking circuits.

A properly operating circuit for your BRAKE lights may cause the HEAD light to dim slightly but NOT become brighter. If the latter is the case, your voltage regulator is again your likely problem. Same scenerio with the TURN signals.
Should you find that a circuit has been "spliced" into another, you will need to isolate the circuits from one another to repair your problem.