I have the drop down bunk, above the driver/passenger seats, so I had to climb up on it, to do it. I'm not sure if it made it easier but I'm no spring chicken, so I think not. LOL As I remember, once the side pillar cover is off, it's pretty easy to pull down the corner of the headliner, a bit, to see the crossbar & get my hand in there. Since the antenna has a magnet, you just put it on top of the bar, between it & the roof.
On a side note, I got the Sirius, from
https://www.crutchfield.com/p_220SXV...0V1-Tuner.html
It $69.99 with free shipping. Sirius offers a $70 rebate (mine also included a 3 month subscription, too), so I ended up be paid 1 penny + free trial subscription, for buying it. It made up for having to crawl around, installing it.
Jeff