If you just want to have a look into the tank then take off the level sender through the First Aid kit compartment (remove the black plastic that holds the kit and then you can unscrew the level sender and have a looksee into the tank). I've found that shining a flashlight in there works pretty well.

congrats on your new car, you'll love the 116 SD. Super smooth ride, extremely comfortable, roomy, (relatively) quiet.

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'79 300SD
'82 Chevy Chevette diesel