Codes PO672, PO673 & PO674. Where are these glow plugs located?

DTC P0671: Cylinder #1 Glow Plug Control Circuit
DTC P0672: Cylinder #2 Glow Plug Control Circuit
DTC P0673: Cylinder #3 Glow Plug Control Circuit
DTC P0674: Cylinder #4 Glow Plug Control Circuit
DTC P0675: Cylinder #5 Glow Plug Control Circuit
DTC P0676: Cylinder #6 Glow Plug Control Circuit
DTC P0677: Cylinder #7 Glow Plug Control Circuit
DTC P0678: Cylinder #8 Glow Plug Control Circuit

Circuit/System Description
The glow plug controller has the ability to perform internal checks, an open or a short to ground on each of the glow plug circuits. If the glow plug controller senses a problem in a glow plug circuit, the glow plug on the affected cylinder will be disabled and the glow plug controller will send an error message to the engine control module (ECM). If this condition exists, the DTC will set.
Conditions for Setting the DTC
The glow plug controller detects that the glow plug control circuit is open, shorted to ground, or has more than 1 ohm of resistance.

Circuit/System Verification
Verify that DTC P064C is not set.
If DTC P064C is set, refer to DTC P064C.
Ignition ON, engine OFF, observe the scan tool Glow Plug 1-Glow Plug 8 parameters. All 8 Glow Plug parameters should closely match each other.
Refer to Testing for Intermittent Conditions and Poor Connections if you cannot duplicate the conditions.
Engine running, observe the DTC information with a scan tool. DTCs P0671-P0678 should not set.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

DTC C0035 Left Front Wheel Speed Sensor Circuit
DTC C0201 Antilock Brake System (ABS) Enable Relay Contact Circuit
The system relay, located within the electronic brake control module (EBCM), supplies battery voltage to all of the valve solenoids and to the ABS pump motor. When the relay contacts close, the EBCM monitors the voltage supplied to the valve solenoids and compares this voltage to monitored ignition voltage.

What Year Make & Model do you have? Don't go & Change Your glow plugs tell you know what the problem is. Don't throw $$ away on something that could be good. Sounds to Me you other problems than glow plugs. Need more info. Was your engine's light on prior to this problem if so there could be some codes to indicate what the problem is!

Error Code: P0404
Fault Location:
Exhaust Gas Recirculation (EGR) System - Range/Performance Problem
Causes:
-Hose leak/blockage.
-Wiring harness.
-Exhaust gas recirculation (EGR) valve/solenoid.
Error Code: P0473
Discription:
Exhaust Gas Pressure Sensor - High Input
Causes:
- Wiring short to positive.
- Exhaust gas pressure sensor (G450).
- Engine control module (ECM).
Error Code: P0674
Description:
Glow Plug Cylinder 4 Circuit - Circuit Malfunction
Causes:
-No Pre-Glow Period.
-Poor Cold Starting.
-Glow Plug Wiring faulty.
-Poor connection.
-Relay.
-Glow plug control module.
-Glow Plug faulty.
-Engine control module (ECM).
Solutions:
Check Glow Plug Wiring.
Check Glow Plug.
Error Code: P0675
Description:
Glow Plug Cylinder 5 Circuit - Circuit Malfunction
Causes:
-No Pre-Glow Period.
-Poor Cold Starting.
-Glow Plug Wiring faulty.
-Poor connection.
-Relay.
-Glow plug control module.
-Glow Plug faulty.
-Engine control module (ECM).
Solutions:
-Check Glow Plug Wiring.
-Check Glow Plug.

Glow plug(s) or glow plug harness

Hello
Here are the meanings of these codes.
P1391 FORD - Glow Plug Circuit Low Input Bank #1
P1393 FORD - Glow Plug Circuit Low Input Bank #2
I think you most likely have a burnt out Glow pug controller. Whenever this controller is replaced all glow plugs should be tested for proper function.

Glowplug relay 180; located in Position 10 in the relay panel. If you unplug the coolant temp sensor, you can trick the computer into thinking it's really cold out, and it will run the glow plugs.

If you have replaced the glow plugs....replace glow plug harness. If code reappears, replace glow plug controller.

FYI: VW numbers the glow plugs backwards. IE: #1 cylinder next to the timing belt cover is number #4 glow plug...so your #2 is actually #3 on the engine.

Above the Relay Panel is the Auxiliary Relay Panel. Has thirteen positions in it.
6 across the top, 7 across the bottom.

Across the bottom the Glow Plug Relay is the fourth one over from the left. It is Number 10.
(180)

0675 is a fault in glow plug # 5 .I dont see a 674 in my list , but I got it off the web and my not be complete. and its for 08 dzl

the glow plg system is monitor when the glow plugs are commanded on for more than 35 seconds and when the battery voltage is between 11.5 ans 14 volts. dtc 1395 or 1396 will set on the bank that is reading less than 32 amps or the the bank that is at least 8 1/2 amps lower than the other bank. Loose fitting pins in the circuit connectors causing high resistance or high resistance glow plugs on the circuit could be the cause. A shorted circuit or low-resistance glow plug(s) on the oppisite bank could be the cause. Each bank needs to draw more than 32 amps but not more than 8 1/2 amps apart. Obviuosly your glow plugs are not the problem since you replaced them all. Therefore, in my past experiences I have found the gaskets and/or the injector harness (under the valve covers) to be the culprits. A simple test you can conduct with an amp meter will help determine if you have a glow plug system that is under performing. First let the vehicle sit for at least an eight hour period. Pop the hood and find the Black/Orange wire going from the solenoid to the glow plug relay. place your amp probe around this wire and turn the key to the on position (do not start the engine). You should have an amperage draw at around 200 amp and it will start to drop as the glow plugs warm up. Now if your initial reading is not close to 200 amps, then you have under performing glow plugs. At this point you will have to ohm out each valve cover gasket and each individual injector harness to determine which one is open. At that point replace the defective culprit and clear your Dtc's then recheck.

Hope this helps,

Ron.