See my readers rig album "gauges"
I installed a street T fitting in the line where the factory sensor is. This T allows the factory and the aftermarket (ISSPRO) sensors to be in the "hot" line comming from the transmission.
There is also a kit you can get that installs in line, but you'd have to CUT into the line and make a nice "rigging".
I also had one in the pan last year. I drilled a hole and welded in a pipe bushing. Now, it simply serves as my drain.
GL