1988 Chev Corvette, what is the best way to clean the plastic dash panels etc ?

Do not use any scratch pads. Rubbing and elbow grease. Fro the plastic, which can fade over time depending on the dun exposure, you may never get plastic back to factory fresh. Use a combination of luke warm water and a dash of Dawn liquid dishwashing soap and a splash of vinegar. Apply with a dish-wiping towel that is soft. Do a little bit at first then using another soft towel, wipe it dry. for the steering wheel, use the same solution but use a soft plastic scrubber for dishes. l use lots of elbow grease and keep rubbing. If it is leather wrapped, there are leather cleaners that can help. Soap will dry out the leather as it pulls out the oils, so a leather treatment is necessary when done. If the crud is on heavy, gently remove some with a toothbrush or even a scrapper, but be careful. There are polishes that may give a shine to the plastic, but there also is spray paints. If you pint the plastic, use the soap combination to remove all the oil and contaminates so it will stick to it. Rinse off the cleaning solution with water and dry it first before painting. (I don't recommend painting buy some do.) The soap solution should work great on the gauge lens, or just warm water and vinegar solution. Wipe dry when done. If you wear glasses, use the glass wipes after or the spray and wipe dry.

Also if you are cleaning the interior dash, use a soft tooth brush to work the cleaning soap solution into the cracks where the plastic ends. Q-tips also work for pre and post cleaning also as it absorbs the moister. Some sprays will make it shine and also collect dust and dirt and start a grimy build up again. The shine on plastic is not like metal. The plastic is dull, the shine is a spray applies at time of manufacture. To respray it, all the old spray has to be removed, using a very fine paste polish, the plastic needs to be smoothed and cleaned.