* Won strength weighs on auto-related shares
* Hyundai Motor falls most since November mileage fiasco
* Hyundai Merchant Marine sinks ahead of share issue
* Solar cell makers rebound on higher polysilicon prices
By Somang Yang
SEOUL, Jan 3 (Reuters) - South Korean shares retreated from nine-month highs after the country's biggest automakers warned of slower sales growth this year as a stronger won erodes their overseas earnings.
The Korea Composite Stock Price Index (KOSPI) fell 0.6 percent on Thursday to close at 2,019.41 points.
Hyundai Motor dived 4.6 percent, its biggest daily percentage drop since Nov. 5, when South Korea's top automaker and its affiliate Kia Motors admitted to false mileage claims in the United States.
The companies aim to increase global sales by a modest 4 percent this year, which would be their slowest growth since 2003, as the won rose to a 16-month high against the dollar.
"Carmakers and their affiliates and suppliers account for 15 to 20 percent of the index, so their fall is dragging down the KOSPI," said Ko Seung-hee, an analyst at SK Securities.
Kia Motors, South Korea's No.2 automaker, shed 3 percent while parts affiliate Hyundai Mobis lost 5.4 percent.
Halla Climate Control Corp, a maker of automobile air-control systems, tumbled 11.4 percent following a media report that its biggest customer, Hyundai Motor, is changing suppliers.
Hyundai Motor said it has switched its air-control parts provider to Japan's Denso for the Sonata sedan made in the United States as part of its "usual" diversification of suppliers.
Shipper Hyundai Merchant Marine tumbled 5.2 percent two days before it is set to issue 11 million new shares worth 196 billion won ($184.30 million).
POSCO was one of the few winners among blue chips, climbing 2.6 percent to extend Wednesday's 3.3 percent gain, buoyed by hopes of recovering steel demand in China.
Among midcaps, solar cell makers Nexolon and OCI were top performers, rising 14.7 percent and 6.9 percent respectively as polysilicon prices climbed.
Overall, advancing shares exceeded decliners 443 to 376.
Foreign investors bought a net 100 billion won worth of KOSPI shares, but they were outweighed by local institutions and retail selling.
The KOSPI 200 benchmark of core stocks closed 0.6 percent lower, while the junior KOSDAQ fell 0.5 percent.
Move on day -0.58 percent
12-month high 2,057.28 14 March 2012
12-month low 1,750.60 19 Dec 2011
Change on yr -0.58 percent
All-time high 2,231.47 27 April 2011
All-time low 93.10 6 January 1981 ($1 = 1063.5000 Korean won) (Additional reporting by Hyunjoo Jin; Editing by Ryan Woo)