"I think what I will do is disconnect the subs and use the test cd to see how low of a frequency the interior speakers can produce good, and base my hpf on that."
Personally, I think that is a really good way to start off. Set all of the levels to zero, turn off any "loudness," "contour," or "super-bass" settings the head may have, and just toy with the crossover frequencies.
Once you bring the subs into the mix, you can tone down the bass to the cabin speakers just a bit more. This allows for more volume, less distortion, as the bass frequencies are what make the speakers travel the farthest, and you do not want *any* of the speakers reaching the extremes of their range of motion.
In most cases, audio tuning is more art than science. After all, you are only trying to please your own ears. Find a good reference album (BT's ESCM is my favorite), and tune it the way *you* like. take your time. You really can't go wrong.
Another thought that just occurred to me. You'll likely want to set the subwoofer crossover frequency on either the sub amplifier, or on the head unit. Not both.