How to hook up air/fuel gauge
 

i was wondering if anybody new a way to hook up a air/fuel gauge up to a 1993 400e?

Here it goes:
If you have a 3-wire O2 sensor, 1 ( usually black ) is the actual signal, varying from 0.1 to 1.0 volts ( engine running ).
The other 2 wires ( usually white ), are ground & 12 volts for the heater.
If you have a 4-wire sensor, they would be 1 signal, 1 - 12 volt & 2 grounds.
You want your A/F gauge connected to the SIGNAL wire.
To determine what's what, ( the colors I mentioned are " generic " ), backprobe ( I use a safety pin for this ) the wires, connect a DVOM and find the correct one for your gauge. ;)

where is the o2 sensor located?

It's located in the exhaust pipe.
Start looking from the engine down.
;)

where would the wire be in the engine bay?