Welcome to the forum
2003 Ranger with a 2.3l, 3.0l or 4.0l engine?
If grounding the SENDER wire, red/white wire, gets gauge to go to HOT then that wire is OK
If sender has two wires, a black/white wire, then repeat test but use that 2nd black/white wire as the Ground, because thats what it is, if gauge doesn't go to HOT again then that black/white wire has a bad ground connection
If sender only has 1 wire then Senders threads to the metal of the engine is its Ground, so DO NOT use tape or sealant on the lower threads or you get a weak or no ground and so incorrect gauge reading
If wires test OK then new sender is bad or new thermostat is the wrong one or bad
Rangers all use a 190-195degF thermostat(since the early 1980's), except the 2001, and up, 2.3l which uses a lower radiator hose thermostat rated at 180degF
The gauge is calibrated for 210degF at 1/2 way point, so just below 1/2 would be correct needle position after 5 to 8min of driving time to full warm up
Also check the heater air, it should be HOT after full warm up, 190degF, not Luke warm, if not then its for sure a thermostat issue, not a gauge issue