Without going into fine detail of the location it is generally at the end of the radiator hose at the engine.
There are two types of cooling system outlet regulated and inlet regulated, both systems have a bypass to WARM the engine then the stat will open to cool the engine down.
Looking at a cooling circuit the system is inlet controlled hence the cooled water returns to the engine, if the stat is stuck open it will ALWAYS run cold.
The cooling system is spec'd to run without the fans and then the fans will come on during no or slow forward movement, the stat can be removed and checked for operation in a sauce pan then heat the water up and see the temperature it opens at, these new stats are designed to fail open.
1 - Connector
2 - Coolant hose, upper
3 - To heater core
4 - From heater core
5 - To connector in coolant hose, upper
6 - Engine Coolant Temperature (ECT) Sensor (G62) Blue in color
7 - Bracket
8 - O-ring
9 - Connector (Always replace)
10 - Bolt: 10 Nm (7 ft lb)
11 - Coolant line
12 - To radiator, upper part
13 - O-ring (Always replace)
14 - To expansion tank, lower part
15 - Bolt - 15 Nm (11 ft lb)
16 - From radiator, lower part
17 - Coolant thermostat
18 - Oil cooler
19 - Coolant or Water-pump
20 - Cooler For EGR
21 - To expansion tank, upper
Radiator Fan Diagrams
1 - Radiator
2 - O-ring (Always replace)
3 - Coolant hose, upper
4 - Connector
5 - Retaining clip
6 - A/C Cut-out Thermal Switch -F163-
7 - From connector on cylinder head
8 - From connector on cylinder head/connector
9 - From ATF cooler (Only for vehicles with automatic transmission)
10 - Electrical connector / Color: black
11 - Cap: Test pressure 1.4 to 1.6 bar
12 - Bolt: tighten to 10 Nm (7 ft lb)
13 - Coolant expansion tank
14 - From connector to radiator for EGR/connector
15 - To cylinder head
16 - Fan shroud
17 - Right coolant fan -V35-
18 - Fan bracket
19 - Retaining clip
20 - Coolant fan -V7-
21 - Bracket
22 - Coolant hose, lower
23 - Electrical connector / Black, 3-pin connector
24 - Coolant Fan Control (FC) Thermal Switch -F18-
* Stage 1. 0N: 92 to 97 C
* Stage 1. OFF: 84 to 91 C
* Stage 2. ON: 99 to 105 C
* Stage 2. OFF: 91 to 98 C
25 - Bracket