Andre,
You are correct. You may go through the firewall or along the hood channel then through the window or door jamb. It should be an easy installation. This link will take you to the owner's manual. Go to page 5 as it's numbered in the manual. Page 7 of the PDF file you are opening.
Meyer Home Plow - Owner's Manual
Step "2b" is what you need...
STEP 2:
WIRING INSTALLATION
Step 2a
Carefully route the Vehicle Harness to the vehicle battery (fi g. 12). You
may need to route thru the grille or behind/under the bumper. Be sure to
stay away from hot, sharp or moving parts of the vehicle and use the zip
ties provided to secure. Attach the Black wire to the negative (-) battery
post (fi g. 13) and the red wire to the positive (+) battery post (fi g. 14).
STEP 2b
Carefully attach the Vehicle Harness Extension to the vehicle harness
(fi g. 15) and route extension into the vehicle. For wireless remote control
models, plug receiver box and secure with zip ties (fi g. 16). The extension
can either be routed thru the fi rewall (fi g. 17) or along the hood channel
(fi g. 18) and thru the window or door jamb (fi g. 19). Be sure to stay
away from hot, sharp or moving parts of the vehicle and use the zip ties
provided to secure.
If routing thru window or door make sure the
hood, door or window closes without pinching or cutting the
harness.
Connect the Controller to the Harness Extension (fi g. 20).
STEP 2c
Install your Receiver Hitch Mount (sold separately, see hitch
requirements on pg. 4) per manufacturers instructions. Measure
center of square hitch to the ground. This dimension (A) should be a
minimum of 10" and a maximum of 13" (fi g. 21). Measure from the
hitch pin hole to the very end of your bumper. This dimension (B)
should be not more than 8-1/2" (fi g. 21). If your dimension does
not fall within these ranges you should contact your hitch dealer/
manufacturer for height or length adjustment accessories.
(A) 10" min-13" max
(B) 8-1/2" max