A soldering iron is a better choice if you want to apply heat carefully, but I wouldn’t recommend heating plastic parts at all if you want them dented. Most of the time they melt, warp and otherwise deform in ways you didn’t have in mind. An Sd.Kfz. 251 Ausf. D front mudguard is a relatively simple shape, you can cut the top part off and rebuild it from thin metal sheet (pewter would probably be ideal, though I‘ve never tried it, but aluminium and annealed brass will work too). This is more work, but will allow you to deform the piece realistically and without fear of anything melting and/or ending up irreplaceably damaged