I would like to mount an aftermarket Cruise Control in my XK8.I would therefore like to hear if anyone knows where I can find a usable speed signal.Preferably at the dashboard as it is easily accessible??.

 

Not sure you can do it like that.
I think all the speed signals to ecu and instruments are over the CAN network.

 

I suggest you review the electrical guide for the 1997 model, if you have not done so
- you can find them in the 'DOWNLOADS' section, for which a link is in the DIY repair and maintenance sticky
https://www.jaguarforums.com/forum/x...ts-data-29800/

It looks like there are at least two options, maybe there are others:
1) monitor the CAN bus signals as baxtor suggests (search for posts by crbass for CAN messages details), and read the ABS module output messages, you could then convert these to whatever is needed using a cheap microcontroller
2) tap into one of the outputs from the main instrument pack which provide speed info - these potentially provide a 12V square wave signal (would need to be verified) with a frequency proportional to speed based on a reading of the electrical guide details - if that is any use with the aftermarket systems you are looking into....
e.g. figure 8.1, connection FC26-6, output, VEHICLE SPEED-PAS, 22 Hz @ 10 MPH (16 KM/H), 44 Hz @ 20 MPH (32 KM)

 

some of the systems come with a pick up to mount near the main drive shaft ...propeller shaft in UK ..easy solution.