Paul__B:
Correct. You will have to construct a test harness with all the connectors for your "tested" harnesses, but with the cabling anchored solidly to an equipment box in which you build the Arduino system.
Forget a "UNO" - very inconvenient to use.
You will want a display - and learn to program that first.
Some sort of keyboard to control it - and learn to program that first.
A number of "port expanders" - you need to research that a little, I do not have time just now.
As 6v6gt points out, you will probably want SD card storage or similar.
In general, a Nano would likely handle it.
Considering it all in summary, I think you need three specific hardware elements.
- An Arduino Nano.
- A number of port expanders to connect to your test harness.
- A cheap/ second-hand PC - readily available, no shortage of these - with its monitor and keyboard, running a Linux operating system such as Mint which is reliable and designed for this sort of development. This fulfils the display, control and storage functions described, perfectly.Whichever way you go, there is a lot of learning and work involved, no way to avoid any of it.

Hello! thanks for the answer, we have loots of computers laying around on the factory, not a problem to install linux on one of them.
about the nano, looks like a good idea (i think will make all what i need)
about programing i have only the basics so i called a guy that worked whith me a long time ago and he is coming friday (he's a very good programmer). i will keep this tread updated whith the difficults i am facing 
thanks in advance.
6v6gt:
For a maximum total of 128 connections, maybe you just need a Nano and SD card reader/writer as already suggested and 8 MCP23017 bus expanders and, say, 128 flying leads.
You may not even need a display if the MCU is always attached to the PC and can display on the serial monitor.
The program would have a learn mode and a test mode.
To use learn mode, you'd need first a drawing of the harness on you would annotate the cable connectors with connections to your cable tester.
This is so that in test mode, you can connect the harness in exactly the same way to the tester.
The learn mode would be essentially this:
User annotates drawing.
User makes all connections between harness and cable tester.
User enters an ID for the harness and maybe the total number of connections made to it so it does not have to assume the maximum of 128.
The cable tester systematically sets a port LOW and all others as Inputs pulled HIGH and builds a map in an array (2 dimensional say 256 x 2 bytes).
At the end, it writes the map to the SD card with a file name derived from the harness ID.
The test mode is very similar, except that it is now simply verifying that the harness matches its map on the SD card and printing a report.
Sample harness:
The learn mode is a basic requirement. The "sample" cable can be given to a customer because it is of no further use in this. What remains is a cable map on an SD card and a drawing so the future harness under test can be connected in exactly the same way that the sample cable originally was.
i would like not to need to have to draw all the cables, it will be massive work and i'm doing it to make things easyer.
i toth to make like connect all the cables the map would make all the pins negative and lith up one by one, but i was thinking if when the program was testing after and a non connected terminal was mistaken put a cable when the program will test it, it will break the arduino (like two positives will be connected togeder ) or a loop going back to the same pin (it happens sometimes)
for all the other guys that are helping whith boards and expander im so glad (its like you all are talking german but my friend will help me figure out what are you talking about)
if my very bad english is not understandable i'm sorry is not my first lenguage.
thanks a lot it will boost my research a LOT.
thanks
Guilherme Reinheimer