I hope you have found it by now. It has been a while since I did this job, but I remember having to cope with a heat shield that curved back and down. My guess is that it is on the back side of the heat shield, meaning you would need to go under the truck and feel on the backside of the shield, if your arms are thin enough. A flexible rod of some sort might help if your arm cannot get up there to find it. If I am remembering correctly, you will be searching between the heat shield and the firewall.
Best of luck, I hope this helps.
whh333