I installed my MS splicing into the stock coolant temp circuit in my '90 and it seems to be working nicely. I have not yet unhooked anything from the oem computer, so the sensor is shared for the time being.
I thought splicing in might affect the signal to the stock ecu and the temp gauge I have wired in, but the gauge indicates the same temps as before for the thermostat opening 180F and the fan coming on 212F (both of which are controlled independently of the oem coolant temp sensor circuit), so everything's good.
I had a bit of a scare when I first used Easytherm. I had the MS installed in the car and clicked to download the new values. I was not expecting easytherm to wipe out the MS's memory and instruct me to open up the case and install the boot jumper
There was no way to cancel, so I just clicked ok to upload (without the jumper installed). Once I removed and opened up the MS and installed the jumper, I ran easytherm again and everything was ok again.
I did a little fooling around with the settings and found that 2700ohms at 86F and 350ohms at 223F got me very close to my gauge readings and ambient temp comparison. I'll spend some time tweaking those settings as I go, and will do dial in the lower temp setting as it gets colder.