oh ok. I miss understood. ok so to summarize, when the shifter is in forward, you cant move the throttles. when they are not in forward the throttles move no problem? so I don't have an answer but, if I remember correctly, the throttles are supposed to be locked when in neutral and free to move in forward and reverse? I don't know for sure since this was broken on my challenger and only some times engaged. any way, when you replaced the old cable did you have to take apart the shifter box at all? it sounds like it is getting forward confused with neutral.
if I remember correctly there are detents in there with ball bearings? if you had to take it apart did you make sure its back together exactly? maybe a one of the disks is in backwards? please forgive me it I am way off base. but I am going by memory and I haven't had my challenger for a number of years