Welcome.
The absolute cheapest way to have a touch screen navi is just buy one and stick it in your window when you need it. The flip up navi screen on the mazda's was a novel idea but not the smartest for a consumer. Im sure if you were to remove the top and the screen of the nav you could buy something and (it would take a lot of work) make it work...but can you easily reach that far if it were a touch screen? Nice Nav's are 100-150 nowadays, not to mention any smartphone has current live gps (traffic etc) standard (android phones do and I believe that apple crap does as well).
I guess it depends what your end goal is and your budget.
For resale, its probably best to leave that part of your car alone and let it be functional and use a 2nd nav.
There is a company on ebay and online that sells a complete new head unit that is a huge touchscreen with nav that works with your steering wheel controls but I dont believe its an easy job.
