1998 406 - Trip Computer - Advice please

I am wondering if anyone can help.

I installed a trip computer (which a got in the breaks yard) in to my 1998
406 LX TD. I know my car wasn’t wired for the trip computer, well not for
all the functions. My car was wired to display Radio & temperature (I had
to add temp sensor to door mirror). For the other functions to work the
trip computer needs a few other wires to receive the needed signals. I did
some homework on this before hand and read the wiring diagrams in the
Haynes book. As the wires only go to and from the instrument panel, I
decided to do some DIY connecting the 4 wires plus 2 to new push button to
substitute for push button on Wash/Wipe stalk). Temperature/ average MPH/
Journey length in miles all work okay, but my problem is fuel economy, MPG
etc does not work. The trip computer does have a reading for the amount of
fuel (in mileage) left in the tank, but it drops this after about 10
minutes into a journey.
My question is; should I give up now as my car needs more than a trip
computer for all functions to work i.e. a higher spec instrument panel or
has anyone seen these problems before with factory fitted trip computers?

 

I've also got a 406 that I've also tried to upgrade with a trip computer.
Can you let me know what you did as I can't get mine to work.

What I've done so far is:
Fitted trip computer, which I plugged into the connector behind the
radio.
Fitted the wiper stalk with the computer function.

I know I still need to fit the temp sensor in the miror. I haven't checked
yet if the wires are there. Can you remmeber how many wires the plug has?
In a breakers yard I've seen two different types.

What I get are three horizontal flashing bars on the display and nothing
else. Doesn't repond to the stalk trip computer button at all.

Am I missing something else?

Thanks
Ryan

 

I've also got a 406 that I've tried to upgrade with a trip computer but I
can't get it to work at all.

What I've fitted is the trip computer and the wiper stalk.
I still need to fit the temp sensor.

What else do I need to fit?