It definitely was available. I had an 83 300SD with Tex.

I remember the 1990 S-Class brochure had samples of both leather and Tex. Everything up to the 420SEL was available with Tex. 560 came with full leather and heated rear seats as standard.

However, in speaking to the dealer in 1990 they said every car they ordered was leather. The 350SDL might have been an exception (they thought) as there were a few customers that wanted the cheapest diesel S class they could get, so a few of those may have come with Tex.

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)