I had what sounds like the exact same squeak except on the driver's side. I could hear it when ever I tapped the brakes or when I accelerated from a full stop. I eventually traced it to the rubber sway bar mount. On the 123 the sway bar runs in the engine compartment along the firewall with a mount behind the brake booster and another behind the battery. You should be able to get access to the passenger side mount by removing the battery. A little shot of lubricant where the sway bar goes through the rubber bushing on the mount should make the noise go away.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold