Ah, awesome -- thank youYou have a 21 XLT so you have a brake booster, I inserted the T fitting between the last valve and the booster itself. I used something like THIS with the small 1/4 nipple facing the fender and just a short 2" piece of line with a slip on cap, as its a vacuum source it won't come off. Easy to pull the cap and insert the straw from the CRC can and inject the cleaner. I cut the booster line at the arrow in the photo for the T fitting.
edit... NOT the arrow on the electrical plug LOL.