76 Spider has a 2+2 formation. I think the back seats were added in 74/75.
Yes, there are Spiders that have a rear seat... These Spiders did not come to the U.S.A., but the "rear seat" does exist. For those who are familiar with a FIAT 124 Sport Spider, the Alfa rear seat looks like the FIAT seat. That is to say that it is a molded cushion that fits over the back area, and it has the stitch pattern that the front seats have. It is not bolted or screwed to the back. It is just held in place by pressure.
Now, as far as this "rear seat" being adequate for a passenger, well, I don't really know. I do know that the FIAT Spider manual clearly states that the car is designed for 2 passengers only. I would think that it is the same for the Alfa Spider. This would be supported by the fact that there aren't any rear seat belts, and not to mention the very limited space... Of course, people do drive with passengers in the "back seat", so this is not surprising. (What's the maximum number of people you have driven with in your Alfa Spider? Mine is 6! Yes, I thought they were all gorgeous and worth the tight squeeze!

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To fit a real seat belt however, would be difficult. The only area in the back that is built to anchor a seat belt is what is already in the car. You could install other anchor points, but I think this would require reinforcing whatever area you chose. (When a roll bar is installed in the rear deck, every anchor point requires reinforcement, because the sheet metal is just too thin.) I would suggest that you try visiting with someone who has experience installing a race harness, and see what they think.
Best regards,