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The bulbs themselves are wired in parallel in your dash, each with their own path to ground. The original rheostat dimmer would be in series ahead of all the bulbs down the two branch lines. If the T5 LEDs are rated for 12V use then the bulbs should be mounted as before. The low voltage LED elements will internally be in series and depending on the number of LED elements may or may not be internally resistored. When mounting the bulbs you will need to pay attention to the bulb +/- orientation in the socket. Depending on the design of the bulb you may find there are noticeable viewing angle differences in brightness and perhaps being too pinpoint for use in dash illumination.
If the bulbs say dimmable then you can put a new dimmer rehostat in series as before, but the rheostat range used with LEDs is often different than for incandescents. Most important for brightness is a zero Ohm rheostat range. The original dimmers often weren't zero ranged and why many people bypassed them (as I did with my '74 140). It may take a bit of trial and error to find an appropriate rheostat to work properly with the combined wattage load. If the bulbs are not listed as dimmable then you can still try using a rheostat and they will normally dim a little bit before blinking out. Lower voltages won't damage the diodes. Depending on the quality of the bulbs the dimming may not be uniform across the dash.
Please post back for others with how you make out, including the bulb brand, description, rheostat used and how happy you are with the lighting and dimming.
Similar wedge bulb bases are used in later dashes and instrument clusters with a rheostat dimmer as well as some 90 degree twist base bulbs and some grossly expensive and proprietary bullet socket bulbs. For wedge bulbs, 5 watts are often specified by Volvo as the originals in dimmable dash lighting, while 3 watts are often listed as the fitment in the aftermarket, the same bulb listed as the fitment for the (non-dimmed) door marker lights which are spec'd by Volvo as 3 watts.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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