1986 K10 speedometer just stopped working

[kxnomore]

My speedometer on a 1986 K10 has never had any troubles until just today, when at 65 mph it just went to zero and won't budge. I looked up the part for the cable and it looks reasonable at just $20. But there was also a speed sensor shown for this year, so I'm curious if maybe the cable could be good and the speed sensor bad? does anyone know where the speed sensor is, or if it serves some other function? Also, how hard is it to replace the cable?
Thanks in advance for any advice, as I'm new to these years trucks.