Hi, No need to take off the rail. Just the injector harness connector.
That is to test to see if the injectors click but the first step is to do the definite ground to the injector harness.
With the injector harness ground wire connected to the battery negative, you know it is definitely grounded, you crank up your car. (you can also run a wire from the battery negative and loosen the ground eyelet bolt, slide it under the eyelet and tighten it down. This will definitely ground not only the injector harness but the entire manifold itself) If it starts, you know it is a ground issue and you will need to add a few more grounds. (Mine has the steel braided ground strap from the intake manifold to the firewall. I put another from the battery negative to the inner fender and a third on the right engine mount from one of the bolts over to the engine bracket). But if when you crank it up, the engine does not fire, then you disconnect your harness connector that you see in the pic. You then run a wire from the positive battery terminal so you can just touch the Positive terminals in the harness connector which go directly to the injector. You should hear a click as you touch the injector Positive terminals one by one. You can touch the injector that is clicking and you will be able to feel the magnet click on and off as you touch the terminal tip. If they click, it means they work. If they do not click, the injector harness has a ground problem inside somewhere that needs to be found or replace the harness. Now, is when you remove the negative wire from the battery negative to see if they work with the way it was originally grounded. Touch them again one by one to see if they click. If they also click that tells you that your grounds definitely are OK and the problem is then with the computer or, wires from the computer to the connector.
When my son's car had this problem, the one in the pic, it gave no codes. It fired with ether or gasoline put into the intake manifold, drove us nuts. We did each of these steps and it ended up being the ground, reason for all the extra grounds on the car but, a neighbor/friend who had the same problem a few years later, was not so lucky and went the next step which is to check for continuity from the injector harness connector to the computer connector... All the injector wires had continuity, in other words, they were good. His ended up being the SMEC