Hmm... odd that you aren't getting any response on SSB...
Is the clarifier locked or unlocked? What kind of output are you getting on SSB, if any? If you're getting output, it might be way off frequency on SSB.
If memory serves me right, this has the same board as the 1-641 base. You can go to
www.cbtricks.com , and go to radio information, then click on "R", scroll down to Royce, and the 1-641 is in there, it has an alignment sheet that you can reference to, if you haven't done so already. Wish I could help you more, but without having the radio on my bench, it's tough to point you in the right direction.
What you could do is setup another SSB rig next to the mobile, and set it up on the same frequency, and use it as a talkback radio to see if you're on frequency or not... I have a small Uniden PC-122XL I use specifically for this purpose. It might be able to help you along. Let us know how you get on.
~Cheers~