There must be a short in the hatchback wiring.
Check all wires and connectors for bare metal contact
Applying the brakes.
Using brakes slows your speed.
brakes, or no sun
My owners manual says 750# w/o trailer brakes, and 1500# w/trailer brakes.
Yes, that will work.
faulty master brake system
check for vacum leak
If you suspect something, get them checked. If you get vibration while applying the brakes, get them checked. If you get grinding noise from the brakes, get them checked.
applying the brakes
Applying the brakes with your foot!
Vibration when applying brakes is most likely warped rotors. Another cause could be loose caliper bolts.
warped rotors(brakes)...most likely front ones