ive had this problem in the past with other apps that want to be on top of the screen.
I hvent tested toaster in the car yet, only on my laptop so it never goes fullscreen.
One thing you can do, is set the skin to be 800x599. It will leave a 1pixel line where your desktop will show through, may or may not be visible depending on your screen and color of your wallpaper.
also in the toaster settings under Display there is a setting "Disable during fullscreen". You may want to try unchecking this (if its checked).
In th future, guino said he was going to add options for RR to be either ON TOP, ON BOTTOM , or NORMAL WINDOW so im looking forward to that for a few other things that i need to be ontop.