89 Lebaron GTC 4 cyl turbo, automatic trans.

89 Lebaron GTC 4 cyl turbo, automatic trans with 3 speed.
I have changed cluster twice and both times odometer doesn't work ( speedometer does) mechanic says it needs a speed sensor in the transmission and it's a 3 hr job. Where is it located and is it all that hard to replace?

Thanks for your time and advice