Been using a pair of them on long leads for years. Just place them anywhere a temperature reading is desired. Be sure to block sources of radiant heat if air temperature only is desired.
(1) National Semiconductor temperature sensor, LM34AH.
http://www.ti.com/lit/ds/symlink/lm34.pdf
https://www.digikey.com/product-detail/en/texas-instruments/LM34AH/LM34AH-ND/5055993
(2) Three-conductor wire. 20 or 22 gauge is fine. Shielding not necessary, but the shield mesh can provide physical protection
(3) Self-adhesive (heat melt glue lined) heat shrink tubing, ?? and smaller
(4) Good digital voltmeter or voltage panel display
(5) Solder tools and supplies
(6) Fuse and connectors as required (current draw is very low)
The output is read with an ordinary hand-held digital multimeter or a digital voltage display. I don't know if they can be calibrated to the Garmin display. The voltage corresponds to temperature, 10mV = 1 degree F. Example: Meter says 2.5 volts. 2.5V is 2500mV. 2500/10 = 250F. Whatever the meter says, just move the decimal point two spaces to the right and you have temperature. (The same sensor is available in Kelvin if that helps.)
Only three connections, aircraft power, ground, and sense. Connecting to the avionics bus so the sensor is ?on? with flight instruments is fine. Meter negative and probe ground should both be connected to the aircraft single point ground bus.